It is well known that, when a load having a capacitive component is connected to a source of power, there is a transient high current flow, or current surge, associated with the charging of this capacitive component to the voltage of the power source. In the absence of any current limiter, the magnitude of the surge is dependent upon the source impedance, which is usually low. With increasingly large capacitances being used, for example for effective supply voltage smoothing purposes, there is an increasingly large energy transient during the current surge.
Where, as is common in electronic equipment, a single power source is used to supply power to several loads, each of which may comprise a switching power supply, circuitry on a large printed circuit card, etc. with its own large smoothing capacitors, the current surge which occurs on connection or switch-on of power to one load can adversely affect the power source to such an extent that reliable operation of other loads is jeopardized. In addition, large current surges impose undesirable stresses on components such as fuses and wiring, and particularly connectors which may be subject to arcing.
Accordingly, it is known to limit the switch-on surge current in various ways. In one known method of limiting the surge current, a resistor with a negative temperature coefficient of resistance and a significant thermal time constant is connected in series with the load. On switch-on of power when the resistor is cold, its resistance limits the surge current, and as the resistor becomes hot in operation its resistance, and hence its power dissipation, falls. However, this simple arrangement is ineffective if the resistor is hot when the power is switched on. Power losses in the resistor, while being reduced in normal operation, may still be unacceptably high.
An alternative is to connect an inductor, which can be designed to provide very little power loss, in series with the load. However, in electronic circuits the necessary size and cost of such an inductor generally make this alternative undesirable.
A further alternative is to connect a resistor in series with the load to limit the current on power switch-on, and to connect a switch provided by relay contacts or a power transistor in parallel with the resistor to bypass it when the switch is closed. Closing of the switch is effected either after a fixed delay which is greater than the current surge period, or in response to the surge current falling below a threshold level. Such an arrangement may involve either an undesirably long delay or undesirably complicated or sensitive control circuitry for the switch.
An object of this invention, therefore, is to provide an improved current limiting circuit for a load including a large capacitance.